Human papillomavirus (HPV) infection is necessary but not sufficient for the development of cervical cancer. Genomic instability caused by HPV may enable cells to accumulate additional genetic abnormalities necessary for carcinogenesis and has been implicated in a causal role in carcinogenesis. Expression of the HPV E6 and E7 oncogenes in primary human keratinocytes (PHKs) leads to polyploidy, which is enhanced by spindle disruption and DNA damage. Previously, it was hypothesized that E6 and E7 induce polyploidy by abrogating the mitotic spindle checkpoint and that E6 degrades the tumor suppressor p53 to induce polyploidy. Our recent studies demonstrate that E6 and E7 do not have a significant effect on the spindle checkpoint. Instead, they abrogate the postmitotic checkpoint to induce polyploidy after microtubule disruption. Interestingly, E6 mutants defective in p53 degradation also induce polyploidy. Moreover, our studies suggest an important role for Cdk4 and Cdk1 in E6-induced polyploidy. In addition, E7 induces polyploidy in response to DNA damage through re- replication, a process of successive rounds of DNA replication without an intervening mitosis. Furthermore, we find that the DNA replication initiation factor Cdt1, whose uncontrolled expression induces re-replication in human cancer cells, is post-translationally modified during E7-induced re-replication. We hypothesize that activation of cdk4 and cdk1 plays an important role in p53-independent induction of polyploidy by E6, modification of Cdt1 is required for E7 to induce re-replication, and E6/E7-induced polyploidy will enhance the progression into aneuploidy and cancer. The specific aims of the proposal are designed to test these possibilities. These studies will shed light on mechanisms by which HPV induces genomic instability and hold promise for the identification of targets for drug development. Infection with human papillomaviruses (HPV) induces warts and is strongly associated with the development of cervical cancer. Modulation of cell cycle checkpoints by the HPV oncogenes E6 and E7 contributes to HPV- induced genomic instability. These studies will shed light on mechanisms by which HPV induces cancer and hold promise for the identification of targets for drug development.